(1) Field of the Invention
The present invention relates to a variable displacement pump for, for example, an internal combustion engine of an automotive vehicle.
(2) Description of related art
Recently, there is an industrial demand for the variable displacement pump to have a two-stage characteristic such that a required discharge pressure is maintained at a first discharge pressure in a first pump revolution speed region and the required discharge pressure is maintained at a second discharge pressure in a second pump revolution region in order to use oil discharged from the oil pump to an equipment having different required discharge pressures such as each sliding portion of the engine and a variably operated valve apparatus which controls a working characteristic of an engine valve.
In order to satisfy the above-described industrial demand, a Japanese Patent Application First Publication (tokuyou) No. 2008-524500 published on Jul. 10, 2008 (which corresponds to International Publication No. WO2006/066405) exemplifies a previously proposed variable displacement pump. In the previously proposed variable displacement pump, the cam ring is installed which is swung overcoming a biasing force of a spring, two pressure receiving chambers are installed at an outer peripheral side of the cam ring, and the discharge pressure is controlled at the two stages by selectively acting the discharge pressure upon these pressure receiving chambers to modify an eccentricity of the cam ring with respect to a rotary center of a rotor.